This invention relates generally to a temperature sensing device for metering fluids, and more particularly to a temperature sensing device or thermowell positioned within a conduit for a flowing fluid medium in which the volume of flow is being metered or measured.
In fluid metering, the temperature of the flowing medium is an important element or parameter in determining accurately the volume of flow. Normally the conduit includes an orifice through which the fluid medium flows and a suitable fluid meter measures the fluid pressure on opposed sides of the orifice. Flow velocity, line pressure and temperature are necessary in order to measure accurately the flow of the fluid medium through the conduit. If the temperature of the fluid medium is not correct, the calculated flow volume by the meter which includes a computer processing unit (CPU) will be inaccurate.
In measuring the gas flow through a gas pipeline, for example, in which the gas flow volume is utilized to effect payment of gas being purchased, an inaccuracy in the temperature of the flowing gas may result in substantial monetary sums being paid or not being paid for delivered gas resulting from an error in the temperature of the flowing gas. Thus, it is highly important that an accurate temperature measurement be obtained for the flowing gas.
A thermowell assembly which comprises a temperature sensing probe (TSP) received within an outer metallic sheath or tube is commonly used for injecting within the pipeline. A thermowell is used so that a temperature sensing probe (TSP) may be easily removed from the thermowell and replaced. Pipelines normally are buried within the ground but a length portion of the pipeline commonly extends above the ground for mounting of a meter to calculate the flow volume of the fluid transported in the pipeline. While the ground maintains a generally uniform temperature, the pipeline section above the ground is exposed to ambient conditions which may vary greatly. Variations in temperature of the exposed pipeline sections results in a temperature conduction from the metallic pipeline to the metallic thermowell assembly which may result in an inaccurate temperature measurement of the flowing medium.
U.S. Pat. No. 4,510,343 dated Apr. 9, 1985 shows a thermowell apparatus having a thermocouple mounted within an outer closed metallic tube or sheath and extending within a pipe. The outer closed tube or sheath is positioned closely adjacent a metallic pipe and an outer metallic housing mounted on the pipe. Thus, temperature from the pipe exposed to ambient conditions is conducted to the outer tube of the thermowell which may result in an error in the temperature of the fluid flow under certain conditions. U.S. Pat. No. 5,048,323 dated Sep. 17, 1991 is directed to a sensor for line pressure and line temperature including a sealed chamber containing a fixed mass of a reference gas. A vibrating quartz density sensor produces an output signal.
It is desirable that a thermowell assembly be provided for a metallic conduit which is substantially temperature insulated or isolated from the metallic conduit so that any temperature inaccuracies of the sensed flowing medium are minimized or eliminated.
The present invention is directed to a thermowell assembly extending within a metallic fluid conduit in a direction perpendicular to the direction of flow to a position past the axial centerline of the metallic conduit. The thermowell assembly includes a temperature sensing probe received within an outer metallic sheath or closed tube. The outer metallic tube which has a closed inner end is exposed to the fluid flowing through the conduit and a plurality of spaced annular ribs or fins extend within the conduit about the outer periphery of the tube for improved temperature conductivity to the temperature sensing probe. The relatively large surface area of the fins improves the rate of temperature transfer by the fins.
The temperature sensing probe includes a temperature sensing element which is received within a thermowell and may comprise various embodiments for the temperature sensing element, such as a thermocouple, a resistance temperature device, a thermometer, a thermistor, and a semiconductor sensor, for example. However, it is understood that other embodiments for temperature sensing elements may be utilized for the present invention and the term xe2x80x9ctemperature sensing probexe2x80x9d as used in the specification and claims herein is to be interpreted for all purposes as including all suitable embodiments of a temperature sensing element received within a thermowell of the thermowell assembly of the present invention for sensing the temperature.
One embodiment of the temperature sensing probe comprises a TSP and tube of a cylindrical cross section with an annular space formed therebetween of a relatively small width or thickness. The annular space adjacent the fins of the thermowell assembly is filled with a temperature conductive liquid for improved temperature conductivity between the outer peripheral surface of the TSP and the adjacent inner peripheral surface of the thermowell assembly. The annular space above the annular fins is filled with air which acts to thermally insulate the TSP from the thermowell assembly.
The metallic mounting structure for the thermowell assembly has a lower externally threaded metal housing for mounting the thermowell assembly onto the metallic conduit. The metal conduit or pipe has an internally threaded metal cap thereon and the lower metal housing is threaded onto the metal cap. Thermal insulating members in many embodiments, such as thermoplastic fittings or members, are positioned between the metal tube and adjacent metallic mounting members so that no metal to metal contact is provided against the tube thereby minimizing thermal increase or loss from the metal tube. As a result, a highly accurate temperature sensing device has been provided.
Also, one embodiment of the invention is designed particularly for high fluid temperatures and does not contain any nonmetallic members therein. A minimal metal-to-metal contact is provided between the metal tube of the thermowell assembly and the adjacent metal mounting structure.
Another embodiment of the temperature sensing probe comprises a temperature sensing assembly mounted within the lower end portion of the internal bore of a finned tube which forms a thermowell within the conduit. The TSP is inserted within the thermowell and held therein by mechanical screw threads through or by a compressive force acting against the TSP. The TSP includes an outer carrier formed of a highly thermal conductive material and a temperature sensing element secured within the carrier by an epoxy material about the sensing element and about a wire connection to the sensing element. Fins on the thermowell tube have an outer diameter slightly less than the diameter of the opening in the conduit wall receiving the thermowell assembly. The thermowell may be fabricated with a uniform stainless steel body, as explained subsequently, for both reducing manufacturing costs and providing even higher accuracy measurements due to the reflective surface of the stainless steel body.
An object of the invention is the provision of a temperature sensing device for metering fluid and is injected within a conduit for a flowing fluid medium to obtain an accurate temperature measurement for the flowing fluid.
An additional object of the invention is the provision of such a temperature sensing device including a thermowell assembly having a temperature sensing probe mounted within a temperature transfer metallic tube which is thermally insulated from the metallic conduit.
A further object of the invention is the provision of a TSP secured within the internal bore of a finned tube and including a temperature sensing element.
Another object of the invention is the provision of a temperature sensing device having a fast thermal response to temperature changes in the conduit fluid.
Other objects, features and advantages will be apparent from the following specification and drawings.